In recent years considerable interest has been shown in combining analog and digital circuits on a single semiconductor chip with particular importance to complementary metal oxide semiconductor (CMOS) analog circuits having very high performance, as required in analog to digital converters. The lack of desirable resistors in CMOS technology has led to increased development in switched capacitor techniques for replacing filters that have been conventionally constructed using discrete resistors, capacitors and inductors. In switched capacitor circuits resistors are replaced by switches and capacitors.
In the analog circuit technology two of the most important circuits are the analog to digital (A/D) converters and analog filters. Both of these types of circuits use a large number of switched capacitor circuits. For example, oversampled analog to digital converters have recently been found to be very useful in achieving high resolution conversion for low frequency signals. The oversampled A/D converters are inherently nonlinear systems which operate on the principle of oversampling and noise averaging. The principal building block of these systems is a fully differential switched capacitor integrator block. This fully differential switched capacitor integrator block is also the main building block of the switched capacitor filters.
In order to achieve high resolution, it is important to minimize noise contribution from power supplies and charge injection from switches. Since fully differential systems are inherently immune to supply noise and achieve a high common mode rejection ratio most high resolution switched capacitor applications use fully differential architecture. However, fully differential architectures require fully differential high gain amplifiers.
High gain fully differential amplifiers require common mode feedback circuitry to maintain the common mode level of output at a predetermined level. Thus, systems using common mode feedback circuitry require that the input differential signal be such that each of the input terminals for a fully differential system have signals applied thereto and these signals should be such that the signal V.sub.IN.sup.+ on the first of two input terminals be equal to the signal -V.sub.IN.sup.- on the second of the two input terminals. For instance, if the input signal V.sub.diff is a sine wave, then when the first half of the sine wave cycle is positive as applied to the first input terminal, the second input terminal should have a sine wave applied thereto with its first half being negative. Accordingly, the input signal EQU V.sub.IN =V.sub.IN.sup.+ -V.sub.IN.sup.-, EQU V.sub.IN.sup.+ =V.sub.cm +V.sub.diff /2 EQU V.sub.IN.sup.- =V.sub.cm -V.sub.diff /2, such that EQU V.sub.diff =V.sub.IN =V.sub.IN.sup.+ -V.sub.IN.sup.-, and EQU V.sub.cm =common mode=(V.sub.IN.sup.+ +V.sub.IN.sup.-)/2.
The use of switched capacitor circuits with differential amplifiers is disclosed in U.S. Pat. No. 4,896,156, filed on Oct. 3, 1988, No. 4,862,121, filed on Aug. 13, 1987, and U.S. Pat. No. 4,574,250, filed on Oct. 13, 1981.
The use of fully differential architectures in designing the high resolution A/D converters and filters provides very desirable results but it also provides the requirement that the input to these systems be a balanced signal around the common mode point of the system. This requirement is generally not compatible in systems where these parts are used. Thus, the choices normally available to the designer using these parts is to design signal conditioning circuitry to precede the high resolution converter on the board. This arrangement requires additional discrete components and very low noise and low distortion design of the signal conditioning circuit block which receives a single ended signal and provides a fully balanced output signal. Such a requirement not only drives the cost up but also places a very difficult requirement on the board designer who wants to design a 16-bit circuit if a 16-bit A/D converter has to be used. Thus, in view of the above mentioned issues, it becomes very important that the input to these high resolution systems be a single ended signal.